Firefox is not installed, so I think your final point is likely to be correct.
apt policy firefox firefox: Installed: (none) Candidate: 57.0.1+build2-0ubuntu1 Version table: 57.0.1+build2-0ubuntu1 500 500 http://au.archive.ubuntu.com/ubuntu bionic/main i386 Packages
Firefox is not installed, so I think your final point is likely to be correct.
apt policy firefox build2- 0ubuntu1 0.1+build2- 0ubuntu1 500 au.archive. ubuntu. com/ubuntu bionic/main i386 Packages
firefox:
Installed: (none)
Candidate: 57.0.1+
Version table:
57.
500 http://